1. Field of the Invention
The present invention relates to a signal size judging apparatus, that is, an apparatus for judging the connection status of the basic signals for a plurality of constituent channels, said basic signals having particular bit rate and frame construction, in a digital transmission system capable of transmitting signals of higher bit rate than the particular bit rate. This can be realized by providing an identification signal indicating connection with immediately preceding basic signal within a frame at the time of synchronous multiplexing after frame alignment of the basic signals for the plurality of channels to connect the basic signals for the plurality of channels within a multiplexed signal.
2. Description of the Prior Art
As an example of the digital transmission system of this kind, the SONET (Synchronous Optical Network) which is to be designated as the standard transmission system in North America will be mentioned hereinafter. In this transmission system, the standard transmission rate and the frame period are set at 51.84 Mb/s and 125 .mu.s, respectively, as shown in FIG. 1. Among 810 bytes (90 bytes.times.8 rows) which constitute one frame, 27 bytes are allocated to the TOH (Transport Overhead) which is designed to administrate the transmission line conditions, and the remaining 783 bytes are allocated to the information bits. Furthermore, as shown in FIG. 2, the frame construction of the signal made by multiplexing N signals constituting the frame construction shown in FIG. 1 is defined as the STS-N frame whose transmission rate is set at 51.84.times.N Mb/s.
However, it is impossible to transmit signals which require a frequency bandwidth of over 50 Mb/s when such frame construction is used. For example, basic signals consisting of three channels shown in FIG. 3A are multiplexed in terms of byte unit as shown in FIG. 3B. Here, H1 and H2 designate the start of the basic signal within the payload. However, a signal having the bit rate of 150 Mb/s cannot be transmitted with the frame shown in FIG. 3B. To enable transmission of signals having a bit rate of over 50 Mb/s, it is conventionally done to connect a plurality of frames within the multiplexed signal by providing an identification signal indicating the connection of the basic signal with the immediately preceding basic signal within a frame. In the case of a signal consisting of three channels, for example, as shown in FIG. 3A, the start of the signal within the payload is designated by the first H1 and H2 and the connection of the first channel with the second and the third channels are designated by the identification signs H1.sup.* and H2.sup.*, respectively.
The multiplexed signal receiver includes an apparatus for judging whether the frame concerned transmits an independent signal or is connected with the immediately preceding frame to allow the transmission of a signal having higher bit rate, that is, the size of the received signals. In order to avoid errors in operation caused by the disorder of the transmission lines, the apparatus for judging signal size is constructed so that it judges that the series of frames are actually connected to each other only when the identification signals of connection are received in succession for a certain fixed period of time (usually of the order of three frames).
FIG. 5 is the block diagram of an example of the conventional apparatus for judging signal size. A quadrupling arrangement is exemplified herein. The received multiplexed signal is separated into the basic signals CH1, CH2, CH3 and CH4 for four channels, respectively, by means of the separating circuit 1. The basic signals CH1, CH2, CH3 and CH4 are introduced into the judging circuit 2, where all of the basic signals CH1, CH2, CH3 and CH4 are judged to be connected together provided that inclusion of identification signals showing the connections is confirmed in each of the basic signals CH2, CH3 and CH4. The signal 6 is then set to a logic 1. The basic signals CH1 and CH2 are introduced into the judging circuit 3, where both of the basic signals CH1 and CH2 are judged to be connected together provided that inclusion of identification signal showing the connection is confirmed only in the basic signal CH2. The signal 7 is then set to a logic 1. The basic signals CH3 and CH4 are introduced into the judging circuit 4, where both of the basic signals CH3 and CH4 are judged to be connected together provided that inclusion of identification signal showing the connection is confirmed only in the basic signal CH4. The signal 8 is then set to a logic 1. Signals 6, 7 and 8 are introduced into the connection status deciding circuit 5 and all of the basic signals CH1, CH2, CH3 and CH4 are judged to be connected together when signal 6 is a logic 1. The signal 9 is then set to a logic 1. Similarly, only CH1 and CH2 are judged to be connected together provided that signals 6 and 8 are both a logic 0 and signal 7 is a logic 1. The signal 10 is then set to a logic 1. Furthermore, only CH3 and CH4 are judged to be connected together provided that signals 6 and 7 are both a logic 0 and signal 8 is a logic 1. The signal 11 is then set to a logic 1.
As explained above, since the conventional signal size judging apparatus is provided with a plurality of individual judging circuits for all possible different-sized receiving signals, it has a disadvantage that as the number of multiplex increases, the number of judging circuits also increases, thereby making the circuit scale larger.